Known test methods and systems for measuring and testing analog circuits on electronic circuit boards require the use of expensive external test equipment and signal node probing mechanisms. During test, the analog circuits operate together functionally while an external analog tester probes, stores and processes analog signals within the analog circuit. These external analog test systems do not provide the capability to easily monitor the analog signals while the analog circuitry operates normally in its system environment due to limited physical access to probe the circuit nodes. Moreover, known analog test methods are not easily used during the entire life cycle of a functional circuit, i.e., during production test, system integration, system test, on-line test or diagnostics, and field support and maintenance.
A particular limitation associated with traditional analog test methods is that the state of the art board designs are so densely populated with integrated circuits that physical probing of the analog signals is very difficult, if not impossible. Additionally, in traditional systems the necessity for external probes to make contact with the signal path often affects the analog signal. Probes add resistive and capacitive loads to an analog signal that potentially cause the signal characteristics (e.g., voltage amplitude, and wave shape) to change when these systems take voltage measurements. As a result, physical probing of analog signals can produce inaccurate measurements and distorted test results.
Yet another problem associated with traditional test methods is that these tests are dependent on the systems and availability of external testers and probing fixtures. These systems usually include complex and cumbersome electronic circuits housed within various cabinets or component chassis. In a field environment, the transportation and upkeep of these components can be a very expensive proposition.
U.S. patent application Ser. No. 07/374,896, entitled "Digital Bus Monitor Integrated Circuit," by L. D. Whetsel, the present inventor, filed on Jun. 30, 1989, and assigned to Texas Instruments Incorporated, ("Whetsel") describes a digital bus monitor used to observe data on a box connecting multiple integrated circuits. The monitor of that application comprises a memory buffer, a signature analysis register, a test port and output control circuits controlled by an event qualifying module. In response to a matching condition, the event qualifying module may perform a variety of tests on incoming data while the integrated circuits continue to operate at speed. The invention of that application further includes a plurality of digital bus monitors that may be cascaded for observation and test for variable width data buses and variable width signature analyses.
The digital bus monitor of Whetsel suffers from the limitation that to be useful for testing analog circuits an external analog to digital converter must be connected to the analog circuitry. Additionally, the digital bus monitor of Whetsel suffers from the limitation that it cannot make comparisons in value between a receive signal and a predetermined test value. For example, if the value of hexagonal A5A5 is anticipated from the tested circuitry, unless the received signal is exactly A5A5 the circuit may not respond. In the analog domain, however, it may be important not only that the signal exactly equal a test set point, but more importantly that the relative magnitude between the receive signal and a test set point may be analytically important. For example, if the analog equivalent to hexagonal A5A5 is the predetermined test set point, if the input signal is greater than the analog representation of A5A5, this information may help isolate the source of the signal variance in the analog circuit being tested.
As a result, there is a need for an analog test method and circuit to measure and test analog signals on circuit board designs that does not require the use of expensive external test equipment and signal node probing mechanisms.
There is the need for an analog test method that can effectively test densely populated analog circuit boards for testing.
There is the need for an analog test method that does not rely on the availability of test equipment in a field environment.
Moreover, there is a need for an analog signal testing method and circuit that does not include the resistive and capacitive loads to distort signal characteristics as is common in traditional analog test methods and circuits.